The present invention relates to a digital mobile radio receiver, especially a receiver using a limiter amplifier.
A receiving unit of a conventional digital mobile receiver, as shown in FIG. 6, comprises an intermediate frequency filter 1 to restrict the band, the a limiter amplifier 2 to amplify and restrict an amplitude for the output of the intermediate frequency filter 1, an orthogonal detector 3 orthogonally detecting and outputting I and Q components, low pass filters 4 and 5 removing useless waves from the I and Q components, A/D converters 6 and 7 each converting an output of the low pass filters to a digital value, and a digital signal processing unit 8 regenerating a received signal.
In the above-mentioned circuit the intermediate frequency filter 1 restricts the band of the input frequency signal IF.sub.0 and provides a signal IF.sub.1 as the output of the intermediate frequency filter 1. Next the limiter amplifier 2 amplifies the output IF.sub.1 and restricts the amplitude thereof, and provides a signal IF.sub.2 as the output of the limiter amplifier 2. Then the orthogonal detector 3 orthogonally detects the output IF.sub.2 and outputs I.sub.0 as the I component and Q.sub.0 as the Q component. Continuously these outputs I.sub.0 and Q.sub.0 are filtered into the outputs I.sub.1 and Q.sub.1 of the low pass filters 4 and 5 after useless waves thereof are respectively removed by the low pass filters 4 and 5. The the A/D converters 6 and 7 convert these outputs I.sub.1 and Q.sub.1 to digital values from the analog values and provide the outputs I.sub.2 and Q.sub.2 as the outputs of the respective A/D converters, and the outputs I.sub.2 and Q.sub.2 are subjected to digital signal processing by the digital signal processing unit 8 to be regenerated
In this circuit, however, if an interfering wave is present in the output IF.sub.1 of the intermediate frequency filter, the limiter amplifier 2 is saturated and the of the amplitudes is restricted by the bigger amplitude of the interfering wave and the desired wave, so that the output IF.sub.2 of the limiter amplifier is always kept at a fixed amplitude.
Consequently, if the amplitude of the interfering wave is bigger than that of the desired one, the limiter amplifier 2 is saturated by the interfering wave and outputs IF.sub.2 without amplifying sufficiently the amplitude of the desired wave. And when the output IF.sub.2 of the limiter amplifier 2 is inputted to the detector 3 in this state, if the detector 3 is of a linear type, the desired wave component of the outputs I.sub.0 and Q.sub.0 of the detector 3 becomes more deteriorated for the insertion loss.
As a result, there has been a problem that if the interfering wave component is removed from the outputs I.sub.0 and Q.sub.0 of the detector 3 by the low pass filters 4 and 5, the input level of the outputs I.sub.1 and Q.sub.1 of the low pass filters 4 and 5, of which the desired wave components are inputted to the A/D converters 6 and 7, absolutely degrades. Accordingly, the effective resolution of the received wave decreases, and a degradation of the receiving characteristic of the receiver occurs.